1. Field of the Invention
The present invention relates to an original reading apparatus for converting original information into an electrical signal and, more particularly, to an apparatus for reading a color original.
2. Related Background Art
A conventional apparatus is known wherein a small color separation filter for a plurality of colors is formed on a light-receiving surface of a solid-state image sensor such as a CCD having a plurality of photoelectric transducer elements. Image signals corresponding to the respective color components and generated by such an original reading apparatus must represent color separation images with good reproducibility. However, if such image signals are used to reproduce a color image in practice (by an electrophotographic system, an ink-jet system, or a thermal transfer system), the hues of the reproduced color image tend to be often different from those of the original color image. In other words, reproducibility of the image signals representing the original hue is poor.
Extensive studies on poor reproducibility have been made, and the following facts are found.
Each photoelectric transducer element of the solid-state color image sensor has good spectral sensitivity for visible radiation as well as invisible light having a long wavelength. A color separation filter of the solid-state color image sensor has a high transmissivity for the visible radiation as well as invisible light having a long wavelength. If an original illumination radiation source emits both visible light and invisible light having a long wavelength, the sensor detects both visible and invisible beams and generates an output representing the intensity of a mixture of the visible and invisible beams. The component corresponding to the visible beam cannot be distinguished from the component corresponding to the invisible beam according to this output. Therefore, the level of the output is different from that of a desired output signal corresponding to only the intensity of the visible beam. In this manner, the sensor detects the invisible light component and outputs the visible light component signal including the invisible radiation component. Therefore, the reproduced color image has different hues from those of the original color image.